search: Ensure that the default result is visible in the scroll view

The default result is set to selected when key focus enters the search
entry. We must also scroll the view if needed.

https://bugzilla.gnome.org/show_bug.cgi?id=726759
iconGrid: Add a key-focus-in signal

This fires whenever a grid's child emits its own key-focus-in signal.

https://bugzilla.gnome.org/show_bug.cgi?id=726759
appDisplay: Ensure the currently focused icon is viewable

We either scroll or paginate to the correct place when an icon gets
key focus.

https://bugzilla.gnome.org/show_bug.cgi?id=726759